The invention relates generally to an arrangement for the spectral analysis of substances. Of particular interest to the invention is an arrangement for use in the determination of the alloying components of metals by spectral analysis.
A known arrangement of this type includes a primary slit and a concave diffraction grating as well as a series of secondary slits which are arranged on a circle. The positions of the secondary slits correspond to the spectral lines which are to be investigated. In operation, a spark discharge is generated between a suitable electrode and a metallic sample to be investigated. The radiation thus produced is directed onto the diffraction grating via the primary slit and the diffraction grating then produces a spectrum or series of spectral lines which are characteristic of the sample being investigated. The intensity of the lines is measured with photographic detectors. The latter generate signals which provide a measure of the proportions of the different elements in the sample.
The spectrum-forming properties of concave gratings are defined by their astigmatism.
In a plane transverse to the lines or rulings of a grating, the image locations lie on a circle, the so-called Rowland circle, when the primary slit likewise lies on this circle. The circle is tangent to the surface of the grating at the center of the concave grating and the diameter of the circle equals the radius of curvature of the concave grating. In a plane parallel to the lines or rulings of the grating, the image points are not clearly formed on the circle and the primary slit must be adjusted so as to be precisely parallel to the lines or rulings of the grating if the resolving power of the grating is not to be reduced.
The currently known spectrometers for determining the compositions of metallic alloys and similar materials, also known as quantometers, and which operate with concave gratings have large dimensions. Accordingly, they are constructed in the form of stationary apparatus for laboratory operation. The large dimensions are, above all, a result of the fact that circle diameters of less than about 1 meter have not been used heretofore. The reason resides in the previously held conception that, if a circle diameter of less than about 1 meter were used, the adjustment of the primary slit would be either impossible or else so complicated that it would only be possible to operate in scientific institutes with suitable specialists.
The adjustment of the primary slit must be effected very carefully and with great precision. In order to accomplish this, the primary slit in the known spectrometers is positioned, for example, on a swiveling arm which is supported on a pivot in the vicinity of the entry window provided for the purpose of permitting the radiation from the spark discharge to travel to the grating for resolution into a spectrum. On the other hand, the primary slit may be mounted so as to be linearly displaceable with a spindle. For the small corrections which are necessary, the large radius of curvature of the Rowland circle makes it possible, as a first approximation, to consider the arc in the region of the primary slit as a linear section. The relatively simple adjustment of the slit which thus becomes possible has, however, associated with it the disadvantage of operating satisfactorily only in spectrometers having a grating of long focal length and, concomitantly, a large Rowland circle.
The large dimensions of the known spectrometers which are mounted so as to be stationary generally do not pose a disadvantage for the spectrometers. The reason resides in that a portion of the spectral lines to be investigated lie in the short wavelength ultraviolet range of the spectrum. Since the short wavelength ultraviolet rays are absorbed by air, the spectrometer itself must be evacuated and for this purpose large auxiliary apparatus is required. Thus, the combination of the spectrometer and the auxiliary apparatus would have large dimensions even if the dimensions of the spectrometer were relatively small.
In those cases where it is intended to analyze substances in order to determine alloying components having usable spectral lines which lie in the visible or long wavelength ultraviolet regions, the auxiliary apparatus for the so-called vacuum ultraviolet are not necessary.
Such an "air device" is not as universally applicable as the "vacuum quantometer" since certain elements cannot be detected. Nevertheless, it offers great advantages in the quality control of materials, both for the producer and the user of the materials. For instance, a producer or a user may be interested in sorting out in a simple manner those bars in a bundle of bars to be delivered which have proportions of alloying components different from the required proportions. However, it is always necessary to bring the individual bars, or discs cut from these bars, to the spectrometer since, as outlined above, the known spectrometers of the type under discussion cannot be transported to the workpieces to be investigated. In particular, it is not possible, for instance, to bring such spectrometers into position above a conveyor belt containing the workpieces to be analyzed so that those workpieces having a composition different from the required or predetermined composition may be registered and sorted out.
One of the earlier-mentioned applications by the same assignee, namely, the application Ser. No. 655,770 filed Mar. 10, 1976 , concerns itself with making it possible to select the diameter of the Rowland circle so as to be small while nevertheless permitting a precise adjustment or alignment of the primary slit to be achieved. A spectrometer formed in this manner may be readily constructed as a transportable device when it operates only the visible and near ultraviolet spectral regions.
To achieve these ends, the just-mentioned application discloses a spectrometric device of the Rowland character which, in a preferred form, includes a housing and wherein a plate provided with the primary slit is movably arranged on the arc of the Rowland circle interiorly of the housing and is mounted so as to be capable of being arrested in any one of a plurality of positions. By virtue of this construction, there is achieved the result that an adjustment or alignment of the primary slit continuously occurs on the circle and parallel thereto. Consequently, it is no longer necessary to select the diameter of the circle to be so large that, as a first approximation, an arc segment thereof may be considered to be a linear section for the purpose of alignment of the primary slit.
In a particularly advantageous embodiment of the foregoing application Ser. No. 665,770 filed Mar. 10, 1976, it is proposed that the plate with the primary slit be guided in two grooves which follow the curvature of the Rowland circle and that the plate be capable of being arrested in the grooves. By virtue of this guidance, an adjustment or alignment along the circle may be insured, on the one hand, while it becomes possible, on the other hand, to achieve a mounting of the slit which is not affected by vibrations.
However, the following problem exists here. In order to generate a spark discharge in the known quantometers, the sample under investigation is placed on a Petrey table and stressed. The sample defines a spark gap with the counterelectrode and ignition is effected by generating a large potential difference across the spark gap. The sample here serves as an electrode of the spark gap. The Petrey table is necessary in order that the spark discharge be produced at a predetermined distance from the primary slit since otherwise an exact formation of the image of the spark gap or spark discharge on the diffraction grating is not possible despite a correct alignment of the primary slit.
Although it is possible, in principle, to integrate the Petrey table in the spectrometric device disclosed in the aforementioned application Ser. No. 665,770 filed Mar. 10, 1976, the handiness of the device, as well as its range of applicability, will be limited thereby. For example, in analyzing the bars contained in a bundle of bar stock which has been delivered to a user or which is to be sent out by a manufacturer, it is necessary, for a device provided with a Petrey table, to cut individual samples from the bars and to stress the samples on the Petrey table rather than being able to perform the analysis without these time-consuming procedures.